1. Field of the Invention
The present invention relates to a toner for developing an electrostatic latent image formed on a surface of a photoreceptor to visualize the image in electrophotography and electrostatic recording, and to a method of preparing the toner.
2. Discussion of the Background
Methods of using a heat roller are widely and conventionally used for fixing methods in electrophotography because of their good energy efficiency. When the methods of using a heat roller is used, recently in particular, a toner is required to have a low-temperature fixability to save energy. Particularly, a toner used in high-speed copiers is more required to have the low-temperature fixability, and trials to reduce heat energy applied to a toner when fixed are frequently made. In addition, a standby (warm-up or recovery) period from a time when an image forming apparatus is turned on to a time when an image can be formed is strongly required to become short to reduce electricity consumption as much as possible.
A technology procurement project of copiers for the next generation is present in DSM (Demand-side-Management) programs of International Energy Agency (IEA) in 1999, wherein copiers producing 30 cpm or more are required to have the standby period not greater than 10 sec and electricity consumption of from 10 to 30 W (dependent on copy speed), which will infinitely save more energy than conventional copiers. As one of the methods to achieve this requirement, a method of having fixing members such as heat rollers have low heat capacity and improving a response of a toner to a temperature can be considered, but the method is not satisfactory.
To shorten the standby period as much as possible, lowering a fixing temperature of a toner is considered to be technically essential. In compliance with the requirement, it is considered that a preset temperature of an image forming apparatus using a conventional low-temperature fixable toner needs to be further decreased by 20° C. Therefore, it is impossible to comply with the requirement with ease even if any known technologies are used, and further advanced technologies are essential.
When a toner has a lower temperature fixability, it is supposed to be difficult to establish a range of fixing temperature (hot offset resistance) and maintain thermostable preservability.
Recently, demands for high-quality images increase in the market, and a toner is required to have a smaller particle diameter because a conventional toner having a volume-average particle diameter of from 10 to 15 μm does not produce sufficient high-quality images. However, a toner having a smaller particle diameter causes various problems such as reduction of a toner amount adhered to a halftone image part of a receiving material such as papers and offset phenomena because a heat quantity from a heating member, which is applied to the toner transferred onto a concavity of the receiving material, is extremely reduced.
To prevent the offset, a release agent such as a wax is typically included in a toner and exuded when fixed, and the release agent needs to be present as a domain so as to easily exude. However, depending on presence status of the domain, the wax present on a surface of the toner particle increases, resulting in deterioration of preservability and developability of the resultant toner. Particularly in a fixer having a low surface pressure, the release agent becomes difficult to exude, and it is quite difficult to complete presence status of the release agent domain in consideration of the fixer having a low surface pressure.
As examples of conventional technologies for a toner having both low-temperature fixability and offset resistance, which prevents adverse effects of a wax included therein, Japanese Laid-Open Patent Publications Nos. 7-295290, 8-234480 and 9-34163 disclose a toner having lower-temperature fixability than conventional toners because of its viscoelasticity. However, the low-temperature fixability is still insufficient.
A toner disclosed in Japanese Patent No. 2904520 and Japanese Laid-Open Patent Publication No. 2000-56511 can be fixed at a low pressure and a low temperature. However, the low-temperature fixability is still insufficient in a fixer having a short standby time.
Recently, Japanese Laid-Open Patent Publications Nos. 2000-89514, 2001-356527, 2002-82484, 2002-162773, 2002-287400 and 2003-351143 disclose a toner having a satisfactory low-temperature fixability and high-temperature offset resistance, wherein THF or chloroform insoluble components in a polyester resin mainly used as a binder resin are specified. However, a binder resin having a high low-temperature fixability although almost insoluble with an organic solvent depending thereon, such as crystalline polyester, is available. Insoluble components with one organic solvent are unable to specify a toner having both low-temperature fixability and high-temperature offset resistance recently required because of having different solubility depending on organic solvents.
Because of these reasons, a need exists for a toner having more low-temperature fixability, high-temperature offset resistance, thermostable preservability and colorant dispersibility than ever; applicable in a fixer saving more energy than ever; and having good developability and producing high-quality images for long periods.